It is thought that adding an ultra-high molecular weight component to polyethylene resins of lower molecular weight may improve the properties of that resin (melt strength, stiffness, etc.) and simultaneously retain the processing properties normally associated with the lower molecular weight PE. However, simple blending of ultra-high molecular weight polyethylene and a linear low density polyethylene may result in both melt and solid phase separation. (See Chen, Y.; Zou, H.; Liang, M.; Liu, P. J. App. Polym. Sci. 2013, 129, 945-953.) Producing an ultra-high molecular weight polyethylene in the same reactor as a lower molecular weight material may be a cost-effective way of creating a bimodal resin with the two molecular weight components already intimately mixed. Thus, if a catalyst capable of producing ultra-high molecular weight polyethylene could be cosupported with a metallocene catalyst such that the two catalysts did not interfere with one another, it is thought that such a resin could be produced in a very straightforward and economical manner. However, selecting catalyst combinations that are compatible (i.e., do not interfere with each others' ability to produce polymer) and produce desired product is a challenge.
Half sandwich chromocenes are disclosed in DE 19710615; WO 2012/040147; US 2013/225820; US 2010/267901; CN 102070732. In particular, J. Organometallics 2000, 19, 388-402 (Dohring, A. et al.) discloses ethylene(cyclopentadienyl) (pyrrolidine)chromium dichloride.
Further, WO 2006/052232 discloses a catalyst comprising a chromocene and bis(n-butyl cyclopentadienyl)zirconium dichloride to produce broad Mw/Mn, high molecular weight polyethylene.
WO 2011/089017 discloses the preparation of high molecular weight polyethylene using a catalyst of a half sandwich indenyl chromocene and a hafnocene.
WO 2008/140175 discloses a combination of metallocene with chromium catalyst on the same support.
Other references of interest include: US 2012/059134, and Macromol. Rapid Comm., 2010, 31, 1359-1363 (d. Kurek, A, et al.).
There is still a need in the art for new and improved catalyst systems for the polymerization of olefins, in order to achieve specific polymer properties, such as impact resistance without negatively impacting the resulting polymer's processability properties.
It is therefore an object of the present invention to provide novel catalyst compounds, catalyst systems comprising such compounds, and processes for the polymerization of olefins using such compounds and systems. Accordingly, it has been discovered that activation of a half sandwich chromocene compound featuring a tethered N-donor, with an alumoxane and consequent supportation on silica produces a catalyst that produces ultra-high molecular weight polyethylene and that further supportation of a catalyst capable of producing linear low density polyethylene produces a 2-catalyst system on the same support, capable of producing polyethylene with a bimodal molecular weight distribution, excellent molecular weight, and good activity.